They came with AC 44S plugs. The onese you buy now will be AC R44S (R stands for resistor--cuts down on noise in the radio from the ignition system). The next step hotter plug is the 45S, but I have found that they just make detonation/spark knock worse on these higher compression engines.
AC Delco no longer makes R44S spark plugs... That is what Nailheads use. I always buy them when I see them. They appear often on eBay. HTH
In my opinion I would spend a couple extra dollars and get a NOS set of r44s, perhaps off eBay. The last # in a spark plug is the heat range - I really think that the "4 " works best on the 401 and on the 300, not to mention that is what the factory calls for... If anyone cares R44S means: R = resistor 4 = 14mm thread 4 = heat range S = extended tip BTW check page 10-2 on the Gran Sport supplement manual --- It also lists AC 42 COM for (long term) high speed operation. I contacted AC Delco a couple years ago asking why they stopped making the R44S plug (many 60's cars used them)--- They told me that there was no demand, and that it was replaced with the R45S. Hope this helps... And here is a really bad pic of an r44s
AC Delco.com The AC Delco website lists the following for a 1966 Gran Sport with 401 engine: R43S.....a colder heat range. Should work, just watch for fouling. Rapidfire #7......a multi-heat range plug. It replaces R43s to R46s Platinum #41-833 There was a discussion on the Yahoo board on which plugs to use....Tom T recommended Bosch Platinum #4236 gapped at up to .050", even for a point ignition. I won't argue with Tom, he knows his nailheads!...
I went with the Bosch 4236 just like Tom Telesco recommended. I wish Tom would post over here more often, he knows Nailheads!
I just put the ol' girl in for another lube and dyno tune to dial in the 850 Q-Jet and haven't had much luck with the NGK's. Of course, no one around here has AC 44s but I was looking at a set on ebay and was wondering how your's were perfroming. Good to know. Have you had them out for a look? What carb are you running?
they are performing excellent,i didnt take them off for a look though,i am running the stock Carter. thanks
I have been running Ac R43s plugs for a few years and about 15000 miles now in my 65 Riv with the 401. Runs great, no detonation, no fouling. Plugs gapped at .035 with a points system.
